Well.. as the title suggests.. truly the cheapest way is by avoiding a hard landing, but let's leave that aside.
This solution cost me about 2 dollars and five minutes.
In this short video I demonstrate how to add rubber caps with foam inserts to both soften a hard landing as well as prevent the legs from sinking into muddy ground, due to the larger footprint.
No glue or screws needed :-)
Hope you like it!